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'Slime worlds' may reflect signs of life

“Slime worlds” may prove excellent targets for the search for extraterrestrial life, according to new calculations. The research suggests future space missions may be able to detect the signature of microbial life around as many as 200 nearby stars.

One of the top priorities for future missions scouting for Earth-like planets is to look for atmospheric components such as oxygen, water, and methane, which could signal life. But these molecules are not always biological in origin, so astronomers believe they can strengthen the case for possible life if these candidates show other intriguing signatures.

One is called the “red edge” – a spike in the amount of near-infrared light emitted from a planet. Plants and some microbes on Earth reflect this light – apparently because absorbing it would cause them to overheat during photosynthesis.

“If our eyes were sensitive to infrared light, vegetation would blind us – it’s as reflective as metal,” says Roger Knacke, an astronomer at Pennsylvania State University in Erie, US.

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Now, Knacke has calculated the spectral signature expected from this effect on planets covered in algae, bacterial mats, or slime molds. He thinks early Earth may have teemed with such life forms – fossilised microbes have been dated to 3.5 billion years ago. In contrast, multi-cellular life only appeared 580 million years ago, and plants sprouted up just 450 million years ago.

Parallel planets

“For 80% to 85% of the time life has been on Earth, it’s just been microbes,” Knacke told New Scientist. “If there’s a parallel on other planets, it takes a long time for multi-cellular life to develop. The signature of these microbes or vegetation might be detectable.”

But detecting it will not be easy. Knacke used models where photosynthesising microbes are as abundant as vegetation is on Earth – an estimate he justifies because no higher life forms are likely to be around to curb the microbes’ growth. Yet even then, such microbes would only increase the amount of near-infrared light reflected from the planet by a few per cent.

“There’s no question it’s going to be a challenging observation,” he says. But he says future space missions such as the Terrestrial Planet Finder-C (TPF-C) may be able to find the signature on planets within about 30 light years from Earth. He says about 200 stars within that distance could potentially support life.

Sample of one

The red edge “seems to be a universal sign of land plants,” says Wes Traub, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US, who will soon become the project scientist for TPF-C.

But he says this is based on a sample size of one – the Earth. “It’s not obvious that if you started up life on a different planet that it would actually discover the chlorophyll molecule and this photosynthetic mechanism,” he told New Scientist. He adds that exotic rocks or molecules on other planets might also be able to reflect light at the wavelengths of the red edge, so a detection of this effect would not prove the presence of life.

But he says TPF-C – which was scheduled for launch in 2014 but may now be delayed indefinitely for budget reasons – is being designed to search for the red edge – along with molecules such as oxygen and water.

“If we could get a really clean signature of that red edge and feel fairly confident we’re not being fooled by strange rocks or other surface materials, it would be a really strong indication of life,” he says.

Knacke is presenting his work on Friday at an astrobiology meeting at McMaster University in Hamilton, Canada.